1. Field of the Invention
The present invention relates to a magnetic sheet, which is capable of suppressing undesirable electromagnetic waves emitted from an electronic equipment, and inhibiting electromagnetic disorders caused due to the interference of the undesirable electromagnetic waves, and also relates to a production method thereof which is efficient and low cost.
2. Description of the Related Art
The magnetic sheets are used for depression of noise, or for RFID. For the purpose of depression of noise, along with rapid developments of electronic equipments, e.g. personal computers and mobile phones, such as down sizing and higher frequency, these electronic equipments suffer from a noise interference due to outer electromagnetic waves and interference of noises occurred within the electronic equipment. In order to suppress such interferences, various countermeasures have been taken for the noises. For example, a magnetic sheet (noise depressing sheet) is disposed adjacent to a noise emitting source or a noise receiving source.
The magnetic sheet is formed by adding alloy (magnetic powder) such as Fe—Si—Al to an epoxy resin, acrylic resin, or the like, and then being cured to form into a sheet by hot pressing. The magnetic powder inhibits noise, and functions as a so-called noise depressor. The better the effect of inhibiting the noise of the magnetic sheet is, the larger an imaginary part μ″ of the magnetic permeability of the magnetic sheet is.
On the other hand, in the case where the magnetic sheets are used for RFID, as a recent trend in the art, radio communication using a coil antenna in an electromagnetic induction method, which is represented as mobile information terminals having IC tags referred as Radio Frequency Identification (RFID), has been widely used. For example, in the mobile information terminal, various conductors (metals) such as a metal casing or metal parts are disposed adjacent to an antenna element for transmitting and receiving due to downsizing of the mobile information terminal. In this case, a magnetic field usable for the transmission is largely attenuated because of the metal present adjacent to the antenna element, and as a result, a communication distance for RFID transmission in an electromagnetic induction system may be shortened, or transmission or receiving of a radio frequency may become difficult because a resonance frequency is shifted. To prevent these electromagnetic disorders, a magnetic sheet is placed between the antenna element and the conductor. As for a function of RFID, it is preferred that the magnetic sheet has a large real part μ′ of the magnetic permeability, and a small imaginary part μ″ of the magnetic permeability.
A curing agent (i.e. a crosslinking agent) for curing a thermosetting organic resin is generally added to a magnetic sheet composition that is a material of the magnetic sheet. The magnetic sheet is hardened by adding the curing agent to the magnetic sheet composition. Especially in the case where a polymer material for use in the magnetic sheet easily absorbs moisture, however, the thickness of the magnetic sheet undesirably changes depending on the fluctuations in the environment such as temperature and humidity. In order to sufficiently cure the magnetic sheet in such environment, the curing temperature is set high and the curing duration is set long. In this case, a large volume of water is consumed as the cooling duration becomes long according to the long and high temperature curing, and moreover the production efficiency decreases. If the curing temperature is set high and the curing duration is set short to solve the aforementioned problem, a problem occurs in the heat resistance of an insulating support.
Moreover, a sulfonium-based cationic curing agent has been commonly used as the conventional curing agent, but the sulfonium-based cationic curing agent contains antimony that is toxic. Therefore, use of such curing agent gives an adverse influence to the environment. Furthermore, as in this antimony-containing sulfonium-based cationic curing agent, a counter ion has a Sb—F bond, which has weak bonding force, a F−1 ion tends to be free. For this reason, in the case where the magnetic sheet containing the antimony-containing cationic curing agent is used around a wiring, the free F−1 ion reacts with water to form hydrofluoric acid, which causes corrosion of the wiring.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2007-95829 describes photocuring of a magnetic binder containing a photocuring cationic curing agent, and discloses an electromagnetic-wave-absorbing sheet having a concentration distribution of magnetic filler from the surface of the sheet towards the depth direction thereof. The curing of this electromagnetic-wave-absorbing sheet is performed by a drum device.